Synthetic lubricating compositions and process for their preparation



Jan. 21, 1958 A. H. MATUszAK ETAL 2,820,815

SYNTHETIC LUBRTCATTNG con/:POSITIONS AND PROCESS FOR THEIR PREPARATION Filed April e, 1954 Alfred H. Maluszak Daniel 8. Maisel' Inventors E. Wendell Carrier By J5- HAN Alorney Unit-Cd Sf2tS, Patent SYNTHETIC LUBRICATING COR/[POSITIONS AND PROCESS FOR THEIR PREPARATION Alfred H. Matuszak, Westfield, Daniel S. Maisel, Union, and E. Wendell Carrier, Cranford, N. J., assignors to Esso Research and Engineering Company, a corporation of Delaware Application April 8, 1954, Serial No. 421,822

4 Claims. (Cl. 260-485) This invention relates to synthetic lubricating compositions and to a process for their preparation. Particularly, the invention relates to complex ester type synthetic lubricants such as those formed from glycols, dibasic acids and alcohols. The invention also relates to a new process for the preparation of these complex esters by use of equipment which obviates many of the technical difficulties of presently used processes.

Complex ester type synthetic lubricating oils prepared by combinations of glycols, dibasic acids, monobasic acids and alcohols are well known in the art of synthetic lubricant manufacture. These esters are constructed from the above compounds on the following general types:

l. Glycol centered complex esters, i. e. alcohol-dibasic acid (glycol-dibasic acid), alcohol il. Dibasic acid centered complex esters, i. e. monobasic acid-glycol (dibasic acid-glycol)z monobasic acid lll. Alcohol-acid terminated complex esters, i. e. monobasic acid (glycol-dibasic acid) I alcohol wherein x is a number greater than zero.

Complex esters are described in detail in U. S. Patents 2,575,195, 2,575,196, and in copending application Serial No. 52,429, tiled October l, 1948, now issued as U. S. Patent No. 2,703,811.

These complex ester materials are prepared by a combination of alcohols, monobasic acids, dibasic acids and glycols in various fashions. Operable alcohols include the following:

Methyl alcohol Ethyl alcohol n-Butyl alcohol n-Hexyl alcohol Z-ethyl butyl alcohol n-Octyl alcohol Z-ethylhexyl alcohol Cetyl alcohol Oleyl alcohol Ethylene glycol mono-mbutyl ether Ethylene glycol mono-2ethylbutyl ether Ethylene glycol mono-Z-ethylhexyl ether Ethylene glycol mono-tert.octyl ether -n-Butylmercaptoethanol lB-tert.-Octylmercaptoethanol ,B-n-Dodecylmercaptoethanol Diethylene glycol mono-n-butyl ether Diethylene glycol mono-Z-ethylbutyl ether Diethylene glycol mono-Z-ethylhexyl ether Propylene glycol mono-butyl thioether Propylene glycol mono-tert.octyl thioether `iropylene glycol mono-n-dodecyl thioether n-Butylmercaptoethoxyethanol tert.-Octylmercaptoethoxyethanol n-Dodecylmercaptoethoxyethanol n-Butylmercaptopropoxypropanol tert.-Octylmercaptopropoxypropanol Propylene glycol mono-nbutyl ether Dipropylene glycol monomethyl ether Dipropylene glycol monoethyl ether Dipropylene glycol mono-n-butyl ether Tripropylene glycol monomethyl ether Tripropylene glycol monoethyl ether Tripropylene glycol mono-n-butyl ether Propylene glycol monoisopropyl ether Dipropylene glycol monoisopropyl ether Tripropylene glycol monoisopropyl ether Many of the above listed ether alcohols, formed by the reaction of ethylene oxide or propylene oxide with aliphatic alcohols, are known in the industry as Dowanols, Carbitols, or Cellosolves.

A group of alcohols especially adapted for use in connection with the present invention are the so-called 0x0 alcohols, prepared by the reaction of carbon monoxide and hydrogen upon the olelins obtainable from petroleum products. Materials such as diisobutylene and C7 oleiins are suitable for this purpose; also higher and lower molecular weight oletinic materials are sometimes employed. The alcohols obtained in this manner normally have a branched chain structure.

Among the monobasic acids which may be employed in the preparation of the esters of the present invention, thel following may be listed as illustrative:

Acetic acid Propionic acid Butyric acid Valeric acid Caproic acid Caprylic acid Pelargonic acid Laurie acid Palmitic acid Stearic acid Oleic acid -Methoxypropionic acid -Ethoxypropionic acid -tert.-Octoxypropionic acid -Ethylmercaptopropionic acid -tert.-Octylmercaptopropionic acid -tert.-Dodecylmercaptopropionic acid Any of the various 0x0 acids Illustrative examples of the dibasic acids which may be employed in the synthesis of the complex esters of the present invention are the following:

Carbon-alkyl derivatives ofthe above acids as, e. g., 2-

methyl adipic acid wherein n is 1 to 26, andthepolyoxypropylene glycols'of the general formula liti Re. l Rijk;

HozoHoHo tononohn whereeither Rlwor R2v isa methyl group andthe; other ishydrogen, and where n is yl. to 20, may-likewise'be employed. Glycolsk containing sulfurV atoms in thioether linkages may also be employed,` and'these include compoundsf as thiodiglycol and 1,2-bis (2-hydroxyethylmercapto) ethane. There also may be usedglycols containing both oxygenand sulfur in similar'linkages; such a compound is.bis2f(.2hydroxyethoxy) ethyl sulfide.

Y The preparation of these complex ester synthetic lubri* cants may be carried out in a one-step or a two-step process, asdesired. The rtwo-step process generally used proceeds. asv follows:

(l)Y PREPARATION OF HALF-ESTER OF GLYCOL AND "DIBASIC ACID In a reaction zone', usually a. glass-lined steel` agitated and jacketed vessel, there is charged the dibasic acid, the glycol, a catalyst, such as sodium acid sulfate, and a water entrainer in the desired amount-s. The reaction is then carried out by heating at reflux temperatures.. until Vthe desired acid number is attained. This ordinarily takes from 2 to l0 hours, depending largely upon the reaction temperature and rate of heat input to the batch. The temperature should not be allowed to exceedabout 400'F. during half-ester preparation. If the reaction tempera ture is allowed to exceed about 400 F., undesirable product formation may result.

The presence of trace amounts ofY oxygen during processing has been found to contribute-to high acidV numbers and corrosivity of the product, so'thata greatV deal of attention must be given Yto exclusion of air. The

reaction vessel is tested for air leakagel before use..

(2) ESTERIFICATION OF THE; HALF-ESTER.

When the half-ester reaction is completed the tempera ture of thereactor is lowered` if.. necessary toadd the desited,l amount of alcoholrapidly. When foaming is encountered it is usually controlled by pressuring with a few pounds of nitrogen. The reactor is then heated to about 400 to 430 F. and the esteriiication is allowed toproceed until completed, usually from about 2 to l0 hours. The temperature here too is critical and should notzbe allowed to exceed about 430 F.

(3) PRODUCT PURIFICA'HON; i

The reaction product is stripped as completely'asjpos.- sible of excess entrainer and low-boiling reactants .at atmospheric pressure. In this stripping step the temperature must again be carefully controlled (below 450 E). If foaming is encountered, itmayv be controlled by using nitrogen pressure momentarily. When the atmospheric stripping is substantially complete, the pressnre'isilowered gradually to -30 mm. Hg absolute pressure andistri'p.- ping continued until a product of the desiredviscosity'is attained. The material is then washed to remove acidic materials and may be Vfurtherstrippedunder vacuum to obtain the desired ash point and viscosity.

In another two-step process the required amount of alcohol is reacted with the dibasic acid to give the acid half-ester. This is reacted with the required amount of glycol to give the crude complex ester which is then treated substantially as described above under product purification.

In the one-step processalll off, the ingredients are comixed and esterified together to a maximum temperature ofv430 F..for fromxZto l0*hours'. The resulting crude complex ester is then treated as described under Vproduct purification.,

Althoughtype. I complexesters have been described, thosenoftypell and-III are prepared by similartechniques.

As was pointed out in the above'description` ofthe preparation of thesecornplex esters, there are several disadvantages inherent in this process. Air leakage into the reaction and strippingzones is veryfdijicult to control. Mixing is usually obtained by a stirrer which must pass througha stung box and. air leakage veryV often occurs at this point.` Asfisthecasefwith batch reactions of this typewhengthe total amount of each ingredient is addedk atone time,.the formation .of undesirable products may result e. g., one mol ot' glycolk may react with one mol of dibasicacid toV forma ring compound. Addition ofthe total amount of alcoholat-one timemay prevent stepwise esteriiication of the half ester andmay increase the pos-y sibility of forming undesirable materials.` Foaming may decrease reactor. capacity and prevent attaining desired;

production, rates., Temperature control is ditlcult andl rates: of heat transfer are relativelylow when the` mass is heated by Wall.A contact. .Diterentvessels are used for different steps due to character of the treatment. An-

other mportant disadvantage'is theepresence of a liquid-` Vhead during low pressure stripping. This limits the amount of unused reactants which can be stripped or Y conversely increases the temperature-to which the product must-be -heatedio-obtaiuaspeciedviscosity. It has now been found, and'forms one-objectfof thisinvention, thatthese disadvantages maybe vovercome by a process involving the useofia circulatingzsystemwherein the product and the compoundsv are continuouslyfcirculated from a reaction zone through ay heating zone and back to the reaction zone.

Asdescribedbelow several important advantages may beobtainedbythe inventive-process. Theser include (1) minimum time,V during which reactants.: must be held at high temperatures, (2) elimination of foaming problems and, hence, decreased stripping and reaction periods, V(3) increased rate of heat transfer which allowsl amore accurate control of temperature and minimizesV start-ing up time, (4) variation as desired in the proportions of reactants present in the reaction zone at one-time to-giveV improved quality of product over that'at-tainable in the 7 conventional system, and (5) substantial eliminationl of I process of this inventiommay involve any ofthe above Vdescribed processes (il e., either-twostep or one-step).

leakage of oxygen into the reaction system;V

The instant process will be more clearly explained by reference to the accompanying drawing. Y In the drawing, Fig. l represents a-'ow plan of the process. The

sulphate, toluene sulfonic acid, sulfuric acid, etc.` is added to the mixture'of entrainer and glycol to increase the rate of reaction.

The mixture is then circulated through lines 4 and 6 Iinto a heating zone 8 by means of a circulating pump 10. The heating zone may be a jacketed reboiler type heating zone wherein the heat exchange occurs through the walls of a plurality of tubes surrounded by the heating medium. Hot oil, Dowtherm, or other liquid heating media may be circulated through the heating zone by means of lines 12 and 14.

After passing through the heating zone, the mixture is passed through line 16 and returned to reactor 2 impinging upon splash plate 18 upon reentry into the reaction zone.

By means of this circulating system, the need for external agitation means, such as a propeller type agitator and its necessary stuffing box is eliminated. The rate of circulation may be controlled by this means so that residence time in the heating zone is as short or as long as may be desired.

When the temperature of the circulating mixture reaches the boiling point of the Water entrainer-glycol mixture, refluxing commences. Entrainer vapors are taken overhead from the reactor through line 22, through the condenser 24 and the liquid passed through line 26 to separator vessel 28. Since at this time there is no water taken overhead, the condensed entrainer is returned through lines 32 and 34a by means of circulating pump 34 to the circulating system.

When reux is well started there is added to the circulating mixture of glycol, entrainer and catalyst, the required amount of a dibasic acid, controlling the rate of addition as desired. Ordinarily 2 mols of dibasic acid per mol of the glycol are required. If the acid used is a liquid, it may be added through lines 3 or 20. lf the acid is a solid, it is` added from storage hopper 35 through line 36. Storage hopper 35 may be equipped with a star feeder, or other means of proportioning solids for ease in addition of the desired amount of acid.

When all or a part of the dibasic acid is added to the circulating mixture and becomes dissolved or melted therein, reaction between the acid and the glycol commences, liberating water. This water is vaporized, entrained by the entrainer vapor and the mixture of vapors taken overhead by means of line 22, condensed to liquid in condenser 24, passed through line 26 and into separator 2S. Here the water separates from the entrainer in a lower layer and may be Withdrawn from the separator vessel through valved line 3). The water entrainer in the upper layer is recirculated to the circulating mixture of reactants as described above. The temperature of the total circulating mixture is adjusted by means of heater 8 to a nal temperature between about 360 and 400 F., and the reaction will ordinarily be completed in from 2 to hours. It is allowed to continue at any event until the desired acid number is reached.

When the reaction between the dibasic acid and the polyhydric material is substantially completed, alcohol at controlled rates and amounts is added through line 20. The alcohol is ordinarily -a branched chain alcohol exempliiied by Z-ethylhexanol or C8 Oxo alcohol. However, any of the alcohols listed above may be used. The alcohol is added in amounts at least suii'icient to completely esterify the reaction product of the dibasic acid and the polyhydric material. 4Ordinarily about 20 to 30% excess of the alcohol will be used.

On addition of all or a part of the alcohol to the circulating stream when esteriiication of the reaction product with the alcohol results, foaming may occur in the system. In this system, unlike the single vessel reaction system of the prior art, foaming actually increases the rate of reaction by virtue of an increase in the rate of circulation and in the rate of heat exchange, and is benecial rather than detrimental to the reaction. The foamY can bebroken-down'in the upper section of' the vreboiler by control of heat input, or upon re-entry into thereaction zone of the stream, it can be removed' by means of splash plate 18.

The circulation of thetotal mixture is continued until the reaction is substantially completed, as evidenced by the neutralization number of the mixture. Ordinarily this reaction will be completed in from about 2 to about 10 hours at a temperature of about 400 to 430 F.

IOnce the reaction is substantially completed, the water entrainer and some lower boiling constituents are rejected from the reaction zone by vaporization below 430 F. and are removed through line 30.

When no more vapor is evolved from the reaction zone at 430 F., the system is placed under reduced pressure by means of a vacuum pump or a steam-jet ejector 37. The circulating materials are gradually subjected to about 10 to 30 mm. Hg vacuum at temperatures no higher than 450 F. to complete the removal of low boiling constituents, and until the desired iinal product viscosity is obtained (2 to l0 hours). The product is removed through valved line 38 and may be washed and treated as outlined above.

lf desired there may be included in the circulating system a iilter for preventing the circulation of solid catalyst with the circulating mixture. This lter is preferable located in reactor 2 at the entrance to line 4 and is adapted to be opened or closed. It may be opened to permit passage of added acid if the acid used is a solid, but closed to prevent solid catalyst circulation once the acid is melted or dissolved. It may also be desirable to equip line 4 with a by-pass system containing a second iilter. Once the reaction is finished and the complex ester product prepared, the bypass line may be used to filter out the solid catalyst so that it may be discarded.

When it is desired to prepare complex esters of type II, that is, complex esters having the formula Imonobasic acid-glycol-(dibasic acidglycol),monobasic acid the procedure may be as follows.

Either the glycol or the dibasic acid is first charged to the reactor along with the entrainer and the catalyst. It will ordinarily be preferred to add the glycol rst. After reaction temperature is reached there is added to the circulating stream the calculated amount of a dibasic acid or glycol, depending upon the material iirst added. After the esterication is completed there is added the desired amount of the monobasic acid, and the reaction carried to completion as described above.

In preparing a complex ester of type III, that is, one having the formula Monobasic acid (glycol-dibasic acid) ,-Alcohol the process may be carried out as follows:

-Either the glycol or the dibasic acid, preferably the former, is charged to the reactor with the catalyst and the entrainer. After reaction temperature is reached an equimolar proportion of the dibasic acid or the glycol, depending upon which reactant was previously charged to the reactor, is added and esterifcati'on carried-out. After this esterication is completed, a molar quantity of a monobasic acid is charged and reacted with the unesteried group of the glycol. Then `the alcohol, in molar quantities, is added to esterify the unesteriiied carboxyl group of .the dibasic acid.

As was explained above, various modifications of the procedures may be made without departing from the spirit of the inventive concept.

An illustration of a preferred embodiment of the process of this invention is as follows:

One molar portion of a polyoxyethylene glycol of a molecular weight of about 200, 30 weight percent of toluene as a water entrainer and a catalytic amount of sodium acid sulfate is introduced into reaction zone from the hopper through valved line 36. The circulation pump 10 is started and the temperature of the circulating mixltuIeisraisecLtoredux-by means of heat exchanger 8.

eration, two mol's'of Sebacic acid arelint'roduced--g'rd allyv into;y thereacti'onzone through line 36 from hopper 35. Circulation is continued and the temperature is raised to" about 365' F. maximum by means of heater '8. When a sample of the circulating stream showsa neutralization-1valueof` about 20G-grams of'KQH/g. of' product, twoum'ols1 of Z-ethyl hexanol .plus abouti a 20% excess is gradually added to the circulating mixture through valvedfline 20. The temperature of the'circulatingrmixture isincrea'sedto 400-430 Circulation is continued until a neutralization number of below 5 g. KOH/ g. is reached.Y It is t'o be understood thatcirculation ofthe" mixture through the heateris continuous throughout the total reaction time.

Once the reaction is completed,L that is when the inal product neutralization number is below about 5 g. KOH/g., ther'material taken overhead through line 22 is rejected through 1i`11e'30.y When all the materialboiling below 430 Fi is rejectedY at atmospheric pressure reduction oft-theV pressurel on the system is begunbyme'ans of 'steam jet ejector 37. The pressure onr the system is gradually lowered to a vacuum of l0 to 30mm. Hg and materialv boiling below 4504 F. a't this pressure taken overhead; condensed and later rejected through line 30y at preferably atmospheric pressure. This stripping operation is` continued until a product of desired nal vise cosity is obtained. The final product isthen withdrawn from the reaction vzone through line 38, cooled, washedl and purilied by methods known to the art.

It has been found in the past that the preparation ofcomplex esters' from oneV mol of a glycol, two mols-of dibasic acid and two mols of an alcohol results in. the formation of lvarying amounts of a diester of the dibasic acid. and theA alcohol. In general, it has been found vthat using polyoxyethylenerglycol'20, sebacic acid and 2-ethyl hexanol about 35- weight percent of di-Z-ethyl hexyl sebacate isformed. Although this formation of the diester is not completely objectionable in all instances,` it isA undesirable since it'v consumes about one-third of the ester ingredients intended for the complex esters.

It has'l now been found and forms a further object of this invention that preparation of a complex ester in the presence of varying amounts-of the diester as a diluent for the reaction results inv an improved complex ester materialin increased yields;

Several preparations of complex esters from polyethylene glycol 200 (l mt), sebacic acid (2 ym.), and Z-'ethylhexanol (2.6m.) were made in the presence of varying amounts of di-Z-ethyl hexyl scbacat'e vas a diluent. Sodium acid sulfate was used as a catalyst forfthe reaction and both one step and two step procedures were used. The resultsv indicatel that Ithe formationl of div esters from the complex-ester starting materials may be greatlyreduced by this method. Set out in tabular form below=are the series of experiments according to the in- Y ventive concept.

Table I.-Eect10f.diester dluent in Ycompleti?. ester prepa# rations Y lIt Willbemotedthat as'the amount of thel diester dilu'ent increasesy from 0% .to 7,0wt. percentl ofV the total diestercomplexester 'ingredient chargerthe weight percent of diester, *prepared from the complex; ester startinglmaterials decreases 'from` 35 to4% =l decrease in utilization' ff CrllpleX eStStItihg' materials ill-ille? ffIljfiOlT of undesirable diesters represents a considerablesavigin manufacturing eos'ts. Y

lt is'also extremely interesting'a'nd unexpected to'nd thatthecom'plex ester materials' prepared as' described above exhibitv amarkedly' improved resistance to lowv temperature,thickening. lt hasL been found in the' past thatcomplex esters of. theftype' contemplated herein exhibit au undesirable-thixot'ropy' upon'st'auding at low temperatures. Since an important use of these complex esters is the lubrication of jet-engines operating at extremely high altitudes where temperatures of aslow as: -50 F. are-encountered, this thixotropic effect isexLV tremely undesirable since it may cause loss of lubrical y tion andfhence engine failure under-these extreme condi tions.

T able` lI.--Low temperature tlzixotropy'0 y complex A=c`omplex ester prepared in absence of diester diluent according to the prior art; n

B =prepared in ther-presence of 15%Vdester as a diluent using one mol o!- polyethylene glycol 200; two mols of sebacic acid vand two mols of 2ethy1 hexa'nol.

It will be noted that the complex ester prepared in accordance with prior art methods show a decided thixotropic etect in blends having a common viscosity. ofv about 8 cs. on being subjected to 40 F. for 25 hours, whereas the complex ester prepared in accordance withV this invention does not change appreciably in viscosity after this treatment.

To illustrate the fact that the materials prepared in accordance with thisV invention have outstanding lubrieating properties, the preparations reported inY Table I were subjected to standard ASTM inspections' for lubri-A cants. The results of these inspections are .set out in` Table Ill.

It will be seen that the ASTM inspection data on the materials prepared in accordance with the instant invention show that these materialspossess propertiesV char-` acteristic of excellent and outstanding synthetic lubri? cants.

If it is; desired, the .diester diluent may be addedl to reactionzonef 2 through valved line13 along" with the polyhydric', material and the water entrainer. It mayalsd be desired to add the diester component through valve'd line 20 to the circulating mixture. Although the data-.reported above shows the-advantages of using from 25% to 75% of diester as' a` diluent, it will be understood that from about'v 5%v to about 95%' of thediest'er,` based on the total weight of the'fdieste'r and the complex` ester ingredients, may also be used.

It is alsol within the concept of this invention Ato prepare the' complex esters as Vdescribed above in the l presence of an excess of a glycol and an excess of adibasic acid. The following examples set'out in detail' the preparation of aseries of esters which are modited by theipresence ofan excess glycolY (1.0m.) and excess sebacicacid (1-.0Ym.)', and 0% and Y30%2 excess, 2-ethy1` l'lexanol.

` Table IIL-Properties of complex esterdester compositions Wt. .Wtu Viscosi cs. at F.

percent percent ty our Flash Fire N eut. Hydroxyl diester diestern V. I pt., F. pt., F. pt., F No. No.

diluent product 210 100 -40 in feed 35 9. 95 51. 1 20, 570 152 -65 495 545 0. 1 4. 0 49 7. 04 32. 9 8, 790 159 75 475 510 0. 14 2. 5 55 6. 33 28. 6 6, 500 162 75 475 515 0. 16 1.3 64 5. 45 23.8 4, 630 167 -75 460 510 0.07 l. 6 74 4. 62 19. 2 3, 040 174 75 470 495 0. 03 l l. 6

COMPLEX ESTER PREPARATIONS [3 m. sebacic acid-P2 m. glycol+2 m. 2-ethylhexanoL] Viscosity, cs. atF. Pour Flash V.I. pt., pt., Neut.

F. E No. 210 100 0 excess 2-eth 1- t7liexanol 28.5 197 10,260 137 -45 535 0.6 30 excess 2-ethyl l'ibexanol 20.4 123 5,535 142 -45 505 0.2

It will be seen that the complex esters prepared in the presence of excess acid and glycol either with or without excess alcohol has lubricating properties such as higher viscosities and tiash points which make them superior or more attractive for certain applications than those made by the prior art method.

To summarize briey, the instant invention relates to improved complex esters and processes for their preparation. Specifically, the invention relates to an improved process for the preparation of complex ester materials having utility as synthetic lubricants. The process comprises the steps of adding to a reaction zone a mixture of polyhydric material, catalyst, a water entrainer and, if desired, a diester diluent, continuously circulating the mixture through a heating zone containing a plurality of tubes around which is circulated a heating medium, returning said heated mixture continuously to a reaction zone, adding in controlled amounts to the circulating mixture the desired amount of a dibasic acid, continuing the circulation until the reaction product of said dibasic acid and said polyhydric material is formed, adding to said circulating reaction product an alcohol, continuing said circulation until said reaction product is substantially completely esteried with said alcohol, increasing the temperature in said heating zone and continuously removing lower boiling constituents from said reaction zone through a condenser and into a separation zone, continuously removing and discarding the water layer from said separation zone, and continuously recirculating the oil layer from said separation zone into said reaction zone until a nal product of desired viscosity is obtained in said reaction zone. If desired the dibasic acid and alcohol may be first circulated, followed by the addition of the glycol or as in a one-step process, all of the ingredients may be circulated initially until the desired crude complex is obtained.

Particularly desirable polyhydric materials are the glycols such as the polyalkylene glycols exempliied by polyoxythylene glycol of a molecular weight of about 200, dibasic acids such as adipic, suberic, azeleic and sebacic acids and alcohols such as iso octyl alcohol, 2-ethylhexanol and other branch chain alcohols containing from about 4 to about carbon atoms.

Although the above description details the preferred embodiment of the invention, a process for the preparation of complex esters of type I, it is also within the inventive concept to produce complex esters of types II and IH. Thus, the process comprises the steps of adding to a reaction zone an organic compound containing at least two esteritlable radicals, an esterication catalyst and a volative liquid hydrocarbon water entrainer, continuously circulating said mixture through a heating zone and back to said reaction zone, adding in controlled rates a second organic compound containing at least two radicals capable of esterifying the radicals of said iirst organic compound in amounts suicient to leave two unesteried radicals present, continuing said circulation and removing water formed until only two unesteried radicals are present, adding to said circulating mixture in controlled rates a third organic compound capable of esterifying at least one of the two unesterifed radicals, continuing said circulation and removing water formed until the esterication of at least one of the two unesteried radicals is substantially complete, adding in controlled amounts a fourth organic compound capable of esterifylng any unesteried radicals, continuing said circulation until all esterication is complete, further heating said circulating mixture, continuously removing water, water entrainer, and boiling constituents from said reaction zone until a final product of desired viscosity is obtained in said reaction zone, and removing and purifying said final product. Thus, according to this procedure, an organic compound containing at least two esteriiable groups, usually either a glycol or a dibasic acid is charged to the reaction zone followed by the second compound capable of esterifying the radicals or groups of the first, usually a glycol or a dibasic acid, depending upon which is charged rst. Subsequently either a monobasic acid, an alcohol, or both is charged to complete the formation of the complex ester of either type I, type II or type III.

The complex ester synthetic lubricants prepared as described hereinbefore may be admixed with mineral oils, either as concentrates or as nished blends. They may also be blended with other synthetic lubricants such as dibasic acid esters, complex esters, polymerized hydrocarbons, glycol ethers, glycol ether esters, formals and the like. They are also compatible with any of the well-known additive materials, and nished blends may comprise a major proportion of these complex esters alone or blended with other lubricants and minor proportions of such agents as viscosity index improvers, pour point depressants, detergent inhibitors, antioxidants, rust inhibitors, extreme pressure agents and the like.

What is claimed is:

1. A process for the preparation of complex ester synthetic lubricants having improved thixotropic characteristics which comprises the steps of esterifying at least one mol of a polyoxyalkylene glycol with at least two mols of an aliphatic dicarboxylic acid selected from the group consisting of adipic, suberic, azelaic and sebacic acid and excess of two mols of an aliphatic monohydric alcohol having from 4 to l0 carbon atoms per molecule in the presence of an esteriiication catalyst and in the presence of from 25% to 75% by weight, based on the weight of the total product, of a C4 to Cm alkyl diester of said aliphatic dicarboxylic acid.

seems-1s 2. A process for the preparation of complex ester` ReferencesrCited-initheilaof this patent UTTE'B STATES PATENTS Pyzel Y Feb. 18, 1941 Strain 1 Ian. 8, 19216 Loder ...Y ..Y ,Aug. 20,1946 Beavers a- Mar. 7, 1950 Smith Nov. 13, 1951v Smithz Nov. 13, 1951?, Ford. Jan. 19, 1954` Kahnetlal Nov. 23, 1954-A Smith Mar. 8, 1955 

1. A PROCESS FOR THE PREPARATION OF COMPLEX ESTER SYNTHETIC LUBRICANTS HAVING IMPROVED THIXOTROPIC CHARACTERISTICS WHICH COMPRISES THE STEPS OF ESTERIFYING AT LEAST ONE MOL OF A POLYOXYALKYLENE GLYCOL WITH AT LEAST TWO MOLS OF AN ALIPHATIC DICARBOXYLIC ACID SELECTED FROM THE GROUP CONSISTING OF ADIPIC, SUBERIC, AZELAIC AND SEBACIC ACID AND EXCESS OF TWO MOLS OF AN ALIPHATIC MONOHYDRIC ALCOHOL HAVING FROM 4 TO 10 CARBON ATOMS PER MOLECULE IN THE PRESENCE OF AN ESTERIFICATION CATALYST AND IN THE PRESENCE OF FROM 25% TO 75% BY WEIGHT, BASED ON THE WEIGHT OF THE TOTAL PRODUCT, OF A C4 TO C10 ALKYL DIESTER OF SAID ALIPHATIC DICARBOXYLIC ACID. 